Implantable medical systems are used to provide stimulation therapy and/or physiological sensing for patients. The implantable medical system includes a stimulation or sensing device that is implanted at a convenient location. Implantable medical leads are routed between the site of implantation of the device and a target site where stimulation or sensing is to occur. Where the route is lengthy, an implantable medical lead extension is used to traverse a portion of that distance.
The implantable medical leads include one or more electrical contacts located near a proximal end of the lead. Where no extension is needed, the proximal end of the lead is physically connected to the stimulation or sensing device so that the proximal contacts of the lead are electrically coupled to electrical circuitry of the device. For scenarios where the implantable medical lead extension is used, then the proximal end of the lead is physically connected to a distal end of the extension where electrical connectors of the extension are coupled to the electrical contacts of the lead. The proximal end of the extension is physically connected to the stimulation or sensing device so that the proximal contacts of the extension are electrically coupled to electrical circuitry of the device. The leads also include one or more electrodes located near a distal end of the leads. Electrical conductors are present within the lead, and each electrical conductor is connected to a respective electrical contact and electrode to provide an electrical path for stimulation and/or sensed signals. Electrical conductors are also present within the extension, and each electrical conductor is connected to a respective electrical contact and distal connector to provide an electrical path for stimulation and/or sensed signals.
Because the lead and lead-extension combination extends over a significant distance within the body, each electrical conductor within the lead and extension is susceptible to receiving extraneous electromagnetic energy that produces electrical current on the electrical conductor. While most ambient conditions expose the lead and lead extension to insignificant levels of such extraneous electromagnetic energy, certain situations may create levels of extraneous electromagnetic energy that are of concern. An example of such a situation is a magnetic resonance imaging (MRI) scan. The MRI scan utilizes a high energy radio frequency (RF) electromagnetic signal. This RF signal may produce relatively large levels of electrical current on the electrical conductor of the lead and extension when the patient having the implantable medical system that includes the lead and/or lead extension combination undergoes the MRI scan. The relatively large electrical current that results from the high energy RF signal produces heating at the electrodes that may create discomfort and even dangerous tissue damage at the site within the body where the one or more electrodes of the lead are located.
It has been found that a shield layer within the lead reduces the amount of RF energy that reaches the electrical conductors, which in turn reduces the amount of current being coupled onto the electrical conductors and reduces the heating at the electrodes to acceptable levels. The manufacturing process of the lead has been altered to include a shield layer when the lead body is being manufactured by creating an inner jacket over the electrical conductor, then creating the shield layer on the inner jacket, and then creating an outer jacket over the inner jacket. The electrical contacts and electrodes are then installed about the inner jacket and are coupled to the electrical conductor to complete the leads. However, leads and extensions that have been constructed without such shield layers or other protective aspects remain vulnerable to the high levels of RF energy of the MRI scan or other situation.